This invention relates to a sealed support leg mounted to a floating roof of a large liquid storage tank. The support leg keeps the roof from traveling downward beyond a certain point while preventing any volatile organic or other gaseous compounds ("VOC's") from escaping upwards from the surface of the liquid through roof penetrations where the support leg is mounted. Without a sealed support leg, the VOC's would disperse in the atmosphere as air pollution. Stringent governmental regulations make it imperative that any loss of VOC's be minimized.
Large, usually round liquid storage tanks may contain a floating internal roof to minimize the loss of liquid products, typically hydrocarbons. These roofs float directly on the liquid surface, and rise or fall with changes in the level of liquid in the tank. Some floating roofs have legs mounted thereon to prevent downward travel of the roof beyond a certain point once the bottom of the leg comes to rest on the floor of the storage tank. These legs typically extend through the floating roof to keep the roof from hitting the bottom of the tank or traveling beyond a certain point when the liquid is withdrawn. Many such legs are slidably adjustable in a substantially vertical plane at right angles to the roof to change the point at which the downward travel of the roof is stopped. However, a continuing problem with floating roof support legs is the inability of such legs to stop or reduce VOC emissions from escaping through or around the top of the leg and thus polluting the atmosphere.
The conventional floating roof support leg comprises an inner support member such as a rod or pipe, and an outer sleeve which is greater in diameter than the inner member. The leg commonly projects through the roof for a certain distance. The height of the leg is adjustable by means of the inner member inserted or slidably mounted into the larger diameter sleeve. Both the sleeve and inner member have a series of holes spaced throughout their length such that a throughbolt can be inserted through the sleeve and member to secure the inner member at a certain fixed length.
The position of the inner pipe relative to the enclosing sleeve results in the accumulation of VOC's in the annular space between the inner pipe and the sleeve above the liquid level in the tank. These VOC's then escape up through such annular space into the atmosphere. Believed exemplary of such conventional support legs are U.S. Pat. Nos. 3,587,911 (Creith), 3,915,332 (Pladys), and 4,018,356 (Szasz). These patents disclose support legs which are not designed to solve the problem of VOC emissions from the tops of such legs.
U.S. Pat. No. 4,243,151 (Bruening) addresses the general problem of VOC emissions resulting from penetrations in floating roofs. The Bruening patent discloses a stationary vertical member, such as a support column or a ladder, penetrating the floating roof of a liquid storage tank. The stationary vertical member extends up through a well, which in turn is surrounded and partially enclosed by a well wall. The well wall is partially above and below the level of liquid stored in the tank, and is separated from the vertical member by a vapor seal. The well wall physically prevents liquid from spilling through the area surrounding the vertical penetration, and the vapor seal within the well wall reduces the space in which vapor from the liquid can first accumulate and then be dispersed into the atmosphere.